For chicken: Mix garlic powder, egg white, water, 1 teaspoon cornstarch and salt to combine. Marinate chicken 20 minutes.
Add 2 to 3 tablespoons of cornstarch to the chicken until it is dry and well-coated. (You may need more or less cornstarch, but make sure the chicken is fully coated and dry; otherwise, it will pop in the oil from the moisture in the marinade.).
Heat oil in pan. Add the chicken pieces, taking care not to crowd the pan and not to overheat oil. You may need to do two batches. Fry chicken pieces until golden brown and drain on paper towels.
Meanwhile, make the sauce. Combine sugar, vinegar, soy sauce, ketchup and chile oil, if using, in a small saucepan over medium-high heat until simmering, stirring to dissolve the sugar.
Mix 1 teaspoon cornstarch and 2 teaspoons water in a small bowl and add, a bit at a time, to the sauce, stirring to thicken. You may not need to use the entire amount.
Combine chicken with sauce, and sprinkle with sesame seeds.